1. Field of the Invention
This invention relates generally to a method and apparatus for boring a hole in the earth and specifically to rotary and percussive drill bit assembly, which is preferably adapted for drilling wells for the hydrocarbon exploration and production industries but may be used for tunneling or similar applications.
2. Description of the Prior Art
The process of drilling a hole in the earth's crust involves abrasive wear of the formation, wherein earth is removed or displaced by hard particles or protuberances on a drill bit forced against or slid along the formation surface at the bore/formation interface. For a solid subjected to a uniaxial stress in the form of a point load (force), the observed behavior of the solid is usually perfect elastic deformation followed by irrecoverable distortion that may take the form of plastic flow or fracture. In other words, the abrasive wear generally occurs by two distinct mechanisms—abrasive wear by plastic deformation and abrasive wear by brittle fracture. Under some circumstances, either plastic deformation or brittle fracture may occur alone, but both often occur together. For both modes, the particle or protuberance must have a hardness greater than the hardness of the material to be abraded. The mechanisms of abrasive wear are treated by I. M. Hutchings in Tribology: Friction and Wear of Engineering Materials, CRC Press (1992).
In abrasive wear by plastic deformation, a hard particle or protuberance affixed to the drill bit is dragged across the surface of the formation under an indentation pressure. The ductile formation flows due to the action of the moving particle. Preferably, the flowing material is deflected, forming a chip which flows up the leading face of the particle in a process referred to as cutting. In cutting, all of the flowing material is removed from the substrate in a process analogous to cutting material with a single point tool in a machining process, such as turning on a lathe. However, abrasive wear by plastic deformation also occurs when the flowing material forms a raised prow of material in front of the leading edge of the particle. Some of the flowing material accumulates in the prow as the indenting particle is dragged across the substrate, while the remainder of the flowing material is ploughed under the particle. Eventually, a portion of the raised prow is lifted up the leading face of the particle and removed—a sequence referred to as wedge formation, which is repeated continuously as the particle moves across the substrate. The abrasive wear by plastic deformation removal mechanisms are exemplified by the use of early rotary drag-type drill bits (which are readily identified by their fish-tall-shaped blades) and present-day diamond-dressed (polycrystalline diamond compact (PDC) or natural diamond) bits.
Contrarily, in abrasive wear by brittle fracture, material removal predominantly occurs by brittle fracture of the formation with little contribution from plastic flow mechanisms. When a solid is subjected to a uniaxial stress in the form of a point load from a hard angular particle which indents the solid, intense shear and compression stresses are induced in the solid at the tip of the particle. At load values less than a critical value (which depends on the hardness and fracture toughness properties of the solid), the induced stresses are relieved by local plastic flow of the solid, densifying the immediate area surrounding the indentation. However, at load values above the critical value, the induced stresses cause a median vent crack to form perpendicular to the surface of the substrate from the bottom of the indention into the substrate. Increasing the point load deepens the crack. After a median vent crack is formed, removing the point load by moving the indenting particle away from the solid results in a relaxation of the deformed material around the point of indentation. Residual elastic stresses in turn cause the formation of one or more lateral vent cracks originating at the median vent crack and curving upwards to the surface of the solid. These lateral vent cracks destroy the integrity of the solid and lead directly to the removal of material from the solid. The abrasive wear by brittle fracture mechanisms are exemplified by the use of percussive early cable tool drilling techniques.
The modern tricone bit, a refinement over the Hughes cone bit introduced in 1908, marries the plastic deformation and the brittle fracture abrasive wear mechanisms in a single bit. As the drill bit axially rotates in the bore under a longitudinal compressive load, the roller cones are forced to revolve around their axes causing the protuberances to rapidly impact the formation (abrasive wear by brittle fracture), but cone offset and friction in the roller cones under load also cause the protuberances to be dragged slightly across the formation for shearing (abrasive wear by plastic deformation). For the protuberances to be effectively impacted against the formation surface, a large axial force must be imparted to tricone bits because the axial force is spread across a large number of indentation points on the roller cones which contact the formation at any given time.
Drilling methods, in which the entire rotary drill bit is periodically axially impacted against the formation during rotation in a manner to aid in fracture of the formation, have been proposed as another means of combining the two modes of abrasive wear to increase drilling rates. Most commonly, the entire cutting surface of the drill bit impacts against the formation due to hammering an anvil surface of the drill bit. The hammer that impacts the anvil surface of the bit can be located at the earth's surface, but it is more commonly located downhole in a drillpipe sub just above the drill bit. Such downhole hammers are usually pneumatically driven from a supply of compressed air at the earth's surface, but hydraulic downhole hammers are also known. Additionally, some down hole impact hammers include a transmission with cams or gears to transfer the rotational energy of the drill string into an axial impact force. Although such systems may use standard off-the-shelf drill bits, because the entire bit is impacted against the formation, the impact force is still spread across the large number of impact points resulting in only a fraction of the overall impact force acting at any given point in the formation.
Regardless of the abrasive mechanisms at play, it has been long recognized by those familiar with the art of drilling oil and gas wells that the most efficient drill assembly is that assembly which transfers maximum energy to the formation (rock face) to aid in the removal of the material of the well bore. Improvements that have advanced the drilling industry to its present day state include increased weight run on existing bit assemblies, increased rates of revolution through advances of down-hole motor assemblies, percussive means on the drill bit assembly, modifications of conventional rotating core bits, improvements to conventional button drag type and PDC drag type bits, and refinements of the mud systems employed. Such improvements are chronicled by the encompassing treatise, J. E. Brantly, History of Oil Well Drilling, Gulf Publishing Company (1971).
Despite such advancements, even today conditions exist in the drilling of deep, horizontal, or high pressured wells where the rates of penetration are very low and the associated costs are high. There is a need for a drill bit assembly that applies additional energy to the rock face (over what is being applied in the industry today) for an increased rate of penetration of the well bore and an accompanying reduction in the cost of the well bore.
3. Identification of Objects of the Invention
The primary object of the invention is to provide a method and apparatus that results in increased drilling rates.
Another object of the invention is to provide a drill bit assembly for which additional energy can be applied to the rock face (over what is being applied in the industry today) that results in a greater rate of penetration of the well bore and a concomitant reduction in well bore cost.
Another object of the invention is to provide a method and apparatus where, by increasing the energy level of the mud system to accommodate the piston mud engine of the invention, the drilling system is allowed to operate as before but with the added energy applied to the rock face that will aid in the penetration rate.
Another object of the invention is to provide a method and apparatus that can be used in any type of mud system including water, oil and polymer systems.
Another object of the invention is to provide a method and apparatus that can be used without modification in conventional rotary systems with ordinary drillpipe, downhole motor systems with conventional drillpipe or coiled tubing systems, top drive systems, vertical wells, deviated wells, and horizontal wells.
Another object of the invention is to provide a method and apparatus that includes a mud engine of the simplest type which uses the technology of the mud pumps that exist today.
Another object of the invention is to provide a method and apparatus that uses a metal spring as an energy accumulator that can be tailored to accommodate varying power directed to the impact bit.
Another object of the invention is to provide a method and apparatus where the mud engine components such as liners, pistons, ports, and valves use state of the art elastomers and hardened wear-resistant materials that typically allow for operation under normal conditions for continuous periods of up to 400 hours.
Another object of the invention is to provide a method and apparatus where the impact bit is of sufficient size and strength to drill and last for periods equaling or exceeding the expected life of the piston mud engine.
Another object of the invention is to provide a method and apparatus where the life of the accumulator metal spring exceeds the other components of the drill bit assembly.
Another object of the invention is to provide a method and apparatus that aids and improves control of directional drilling.